The general configuration of a conventional hydraulic speed change gear 40 is shown in FIG. 5. A variable capacity type hydraulic pump 41 (hereinafter referred to as "the pump 41") and a pump 42 for a servo valve (hereinafter referred to as "the servo pump 42") are driven by a power source (not shown). The pump 41 is connected to a hydraulic motor 43 via hydraulic circuits 44 and 45. The pump 41 selectively discharges pressurized oil to one of these hydraulic circuits so as to enable the hydraulic motor 43 to turn in the forward direction or in the reverse direction. The maximum pressures of the respective hydraulic circuits 44 and 45 are limited by the relief valves 46 and 47; and the occurrence of negative pressure is prevented by the suction valves 48 and 49. The hydraulic motor 43 drives a travel drive wheel (not shown). The servo pump 42 is connected to a servo cylinder 53 via a conduit 51 and a servo valve 52, which controls the discharge of the pump 41. The servo valve 52 includes springs 52a and 52a at opposed end positions, and the servo cylinder 53 also includes springs 53a and 53a at opposed positions. When the servo valve 52 is not in operation, a servo valve input lever 57 (hereinafter referred to as "the link lever 57") is in a position where the tilt rotational angle of the pump 41 is zero. As the servo valve 52 moves according to the movement of the link lever 57, the servo cylinder 53 moves to its middle position between its left position D and its right position E; and the pump 41, which is interlocked with the servo cylinder 53, does not discharge pressurized oil. Hence, the hydraulic speed change gear 40 is at rest.
Connected to the servo valve 52 and the servo cylinder 53 are links 55 and 56, and the link lever 57. The link lever 57 is connected via a wire cable 58 to a travel operating lever 59, which is provided at a driver's seat (not shown).
The operation of the hydraulic speed change gear 40 will be described. When an operator operates the travel operating lever 59, the link lever 57, the link 55, and the servo valve 52 are operated via the wire cable 58. When the link lever 57 moves in the direction of the arrow from J to M about a fulcrum L relative to the link 56, a fulcrum K of the link lever 57 and the link 55 moves in the direction of the arrow from K to N accordingly, and the servo valve 52 shifts from its position A to its position C via the link 55. This causes the pressurized oil from the servo pump 42 to flow to the left position D of the servo cylinder 53, the fulcrum L to move in the direction of the arrow from L to R, and the tilt rotational angle of the pump 41, interlocked with the servo cylinder 53, to increase, thereby causing the pump 41 to discharge pressurized oil. As the link lever 57 moves back in the direction of an arrow from N to K, the servo valve 52 shifts from its position C to its center position A to cut off the flow of pressurized oil from the servo pump 42 to the servo cylinder 53, causing the servo cylinder 53 to stop in its current position (the position to which the fulcrum L has moved). In other words, the pump 41 discharges the amount of flow which corresponds to the shift amount of the travel operating lever 59. Thus, a vehicle travels at a predetermined speed which corresponds to the operating amount of the travel operating lever 59.
On the other hand, to bring the vehicle to an emergency stop in case of an emergency, the travel operating lever 59 is set back to the neutral point, thereby shifting the servo valve 52 from its position A to its position B. This causes pressurized oil from the servo pump 42 to flow to the right position E of the servo cylinder 53, and the pressurized oil on the side of position D to drain to a tank 60 via the servo valve 52. Thus, the tilt rotational angle of the pump 41, which is interlocked with the servo cylinder 53, is reset so that the pump 41 no longer discharges pressurized oil and the vehicle stops.
If the travel operating lever 59 is directly connected to the servo valve 52, then its operation is relatively satisfactory. Generally, however, the servo valve 52 is directly connected to the pump 41 or is disposed in the vicinity of the pump 41 to simplify the piping connection. Moreover, the pump 41 is disposed in the vicinity of a power source, so that the servo valve 52 cannot always be directly connected to the travel operating lever 59, which is located at the driver's seat. In such a case, the wire cable 58 is generally used to join the travel operating lever 59 to the servo valve 52. In construction equipment, agricultural equipment, etc., which handles stone, earth, or sand, there are cases where stone, earth, or sand is caught in the wire cable 58, and an operation failure takes place, leading to an extremely hazardous situation and posing a problem in that the vehicle cannot be stopped in case of an emergency.
There is another problem in that the time for stopping the vehicle differs according to the speed at which the operating lever 59 is reset. Hence, it is possible to achieve a quicker reset by increasing the tension of the springs 52a of the servo valve 52; however, this method poses a problem in that a higher operating force is required.